Transforming growth factor β (TGFβ) is a cytokine that plays an important role during normal embryogenesis due to its multifunctional effects on cellular responses such as proliferation, differentiation, apoptosis, and migration. TGFβ has during recent years become recognized as a potent regulator of cellular plasticity, which is a central event during embryogenesis and tumor progression. TGFβ signals through its binding to the type II and type I serine/threonine kinase receptors (TβRII and TβRI, respectively), resulting in their hetero-oligomerization, which subsequently activates various intracellular signalling pathways. For example, TGFβ activates the kinase activity of TβRI to phosphorylate the latent transcription factors Smad2 and Smad3 in early endosomes, which induces complex formation with Smad4 and nuclear translocation, allowing regulation of target genes. Sorrentino et al., Nat. Cell Biol. 10:1199-1207; 2008. In addition to the Smad pathways, TGFβ/TβR signaling can also activate non-Smad pathways.
TβRI has been found to harbour a consensus binding site for the ubiquitin ligase tumour necrosis factor receptor (TNFR)-associated factor 6 (TRAF6), which was initially identified as mediating the activation of NF-κB by Interleukin-1. Cao et al., 1996. When bound to TGFβ, TRAF6 activates the TGFβ activated kinase 1 (TAK1), which in turn activates p38 mitogen activated protein (MAP) kinase pathway. Yamashita et al., Mol. Cell. 31:918-924; 2008; and Liu et al., Mol. Cell. 35:26-36; 2009. TRAF6 was also known as an E3 ligase, which was reported to interact with TβRI (also known as activin like kinase (ALK) 5) at a highly conserved consensus motif. Sorrentino et al., Nat. Cell Biol. 10(10):1199-1207; 2008. The TβRI-TRAF6 interaction leads to TGFβ induced TRAF6 autoubiquitination and Lys63-dependent polyubiqitination of TAK1. The activated TAK1 in turn activates MKK3/6 leading to p38 activation and resulting in apoptosis. Thakur et al., Future Oncol. 5(1):1-3; 2009 and Landstrm et al., Int. J. Biochem Cell Biol. 42(5):585-589; 2010.
Posttranslational modifications of TβRI, such as monoubiquitination or Lys63-linked polyubiquitination, have emerged as an important mechanism to control the localization or function of this protein, whereas Lys48-linked polyubiquitination of TβRI was originally described to instead target its substrate for proteasomal degradation (Hershko and Ciechanover 1998, and Ikeda and Dikic 2010). TRAF6 is known to induce Lys63-linked polyubiquitination of its substrates, including TAK1. Yamashita et al., 2008.